Sexual transmission of HIV is relatively inefficient. Most sexual exposures do not lead to transmission. Of the swarms of virus in the semen, only a single virion is usually transmitted. In SIV transmission to female macaques, which is the basis for current models of HIV transmission to women, infection propagates in a small focus of genital tissue CD4 T cells. In the first week, SIV is restricted to the genital mucosa where the virus amplifies before systemic dissemination, providing a window of opportunity where intervention could inhibit transmission. This proposal will explore HIV transmission in 2 human models - polarized human cervical explants and humanized mice transplanted with fetal human hematopoietic stem cells, liver and thymus (BLT mice). Our first aim is to characterize the cell types, phenotype and tissue distribution of infiltrating hematopoietic cells in normal human cervix and compare it with the distribution of human cells in the BLT model. This will provide important information about how closely the genital mucosa of BLT mice resembles humans. The second aim is to characterize HIV infection in cervical explant cultures and in BLT mice infected intravaginally to identify the first infected cells and understand how the infection amplifies and spreads in the tissue. In addition we will characterize the changes in immune cell activation and induction of cytokines, interferons and chemokines that occur in both models following HIV infection. These changes are hypothesized to enhance HIV susceptibility of resident cells in the mucosa and promote recruitment of additional susceptible cells. HIV infection of macrophages and T cells does not trigger cytosolic innate immune nucleic acid sensors to induce Type I interferons. HIV evasion of these sensors is mediated by the Trex1 DNase, which digests failed reverse transcripts. The lack of efficient HIV replication in dendritic cells also effectively suppresses interferon production by most dendritic cell subsets. The main source of Type I interferons in chronic HIV infection, plasmacytoid dendritic cells, are not normally found in the uninflamed genital mucosa. Our underlying hypothesis for aim 3 is that the lack of an immediate interferon response allows HIV to gain a foothold in the tissue. We hypothesize that knocking down TREX1 in CD4+ cells in the genital tract will trigger an IFN response to HIV in infected T cells that will suppress the local burst of viral replication. Howeve, early IFN induction might also enhance inflammation and recruitment of immune cells to the genital mucosa that could have an opposite effect. Therefore in vivo studies are needed to decipher the effect of interferons at the point of transmission. In aim 3, interferon levels will b manipulated around the time of intravaginal HIV exposure by treatment with exogenous interferon?, knockdown of TREX1 and other genes involved in interferon expression, and antibodies that neutralize interferon activity. We will focus on analyzing the effect of local TypeI interferon induction on HIV replication, the activation status of CD4 T cells in the genital mucosa, the recruitment of immune cells to the tissue and the expression of cytokines and chemokines and chemokine receptors that regulate immune cell ingress and egress from the tissue.